1. Field
The present invention relates to methods of inspection usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to determine features of the substrate, such as its alignment relative to previous exposures, a beam is reflected off the surface of the substrate, for example at an overlay target, and an image is created on a camera of the reflected beam. By comparing the properties of the beam before and after it has been reflected off the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties.
A property of a substrate that is monitored is the alignment of the substrate before it is exposed. A substrate will undergo several iterations of being covered in resist, exposed and processed to remove the unexposed resist. Each time a new layer of resist is applied to the substrate in preparation for exposure, the substrate must be aligned properly so that the new layer of resist is exposed in the same places as the previous layer of resist, to ensure that the resulting pattern is as sharp as possible. The method of ensuring the alignment must be precise, but to ensure throughput of the substrates is not compromised, the overlay method must also be quick.
The state of the art describes the use of overlay markers on the surface of the substrate. Each time a layer of resist is applied, an overlay marker is exposed, etched or otherwise created on the layer of resist and this overlay marker is compared with the marker on the substrate surface (or on the layer below it if the previous layer of resist has not been removed at that point). The overlay marker will often take the shape of a grating. The overlap of one grating on another is detectable using an overlay radiation beam by measuring the diffraction pattern of the beam as it is reflected from the surface of the superposed overlay markers.
However, these sorts of overlay markers detect only the relative position of one bar of a grating (i.e. the width of the pitch of the grating) with respect to the position of the bar underneath it. If the gratings are misaligned by more than the width of a grating pitch, there is no way, from the diffraction pattern, to determine this. In other words, misalignment below a certain threshold (the width of the grating pitch) can be measured, but errors over this threshold may be easily missed. Of course, large errors in alignment cause large errors in the exposed pattern and the substrates with these mis-exposures will often need to be discarded or completely stripped and redone.